1. Field of the Invention
This invention relates to the production of tetraalkyl silicates, particularly tetraethyl and tetramethyl silicates.
Alkyl silicates can be made by the first reaction of metallic silicon with the corresponding alkanol. This route requires either high temperature, high pressure, high surface area of the metal, or a combination of these to achieve a practical reaction rate.
Silicon halides can be caused to react with alkanols to make organosilicates. The process produces large amounts of hydrogen chloride which is corrosive and presents a disposal problem.
Electrolytic methods may be employed to produce tetraalkyl silicates. The electrolytic process requires a large capital expenditure, has high operating expenses, and may involve difficult problems of separation.
Another route to tetraalkyl silicates involves transesterification, wherein a high-boiling alcohol reacts with silicon to form an intermediate silicate which then reacts with a lower boiling alkanol to form the desired tetraalkyl silicate.
2. Description of the Prior Art
Anderson et al in U.S. Pat. No. 3,803,197 granted Apr. 9, 1974 disclose a process for preparing alkyl and 2-alkoxyethyl orthosilicates by reacting "simple alcohols" or 2-alkoxyethyl alcohols containing 1-4 carbon atoms in the alkoxy group with silicon in the presence of a catalytic amount of an alkali metal salt of a 2-alkoxyethyl alcohol. This patent teaches preparation of a catalytic alkali metal alkoxide from the metal. Anderson et al apply their invention to both the synthesis of higher boiling orthosilicates and lower boiling silicates.
The preparation of tetraethyl orthosilicates from a source of silicon suspended in tetraethyl silicate and ethanol, catalyzed by a large amount of sodium ethoxide and cocatalyzed by either an ether alcohol or an alkanolamine, is described by Flick et al in British patent application No. 2,018,800A published Oct. 24, 1979 (cf. West German patent application No. 2,816,386 published Oct. 25, 1979).
A process for synthesizing tetraalkyl orthosilicates by contacting a source of silicon with an alcohol and its corresponding alcoholate is the subject of U.S. Pat. No. 4,113,761 granted Sept. 12, 1979. The process of this disclosure requires additionally either a surface active agent, sodium methylate, or the use of pressure (cf. British patent applications No. 1,559,335-6-7).
Bleh et al in U.S. Pat. No. 3,627,807 granted Dec. 14, 1971 disclose a method for preparing tetraalkyl silicates from an alcohol and the corresponding alcoholate in the presence of a large amount of the same tetraalkyl silicate ester. The patent discloses stringent limitations on the ratios of the materials in the reaction mixture and all three compounds (the alcohol, the alcoholate, and the ester) are restricted to the same alkyl group.
U.S. Pat. No. 2,927,937 granted Mar. 8, 1960 to Gaines sets forth a method for preparing alkyl orthosilicates by attrition milling of a silicon source in the presence of an alcohol and an alkaline catalyst, preferably under pressure. One of the five preferred catalysts disclosed by Gaines is sodium ethoxide. Extended ball milling is required for this process.
3. Objects of the Invention
A principal object of this invention is to provide a method for preparing tetraalkyl silicates.
Another object of this invention is to lessen the formation of by-products, especially unsymmetrical mixed esters, in the preparation of tetraalkyl silicates.
Still another object of this invention is to provide a safe and convenient method for preparing potassium alkoxides and potassium glyoxides suitable as catalysts for the reaction of silicon metal and alkanols to make tetraalkyl silicates.
Another object of this invention is to provide a clean surface of silicon at all times during the reaction.
A further object of this invention is to provide a process for the preparation of tetraalkyl silicates which is amenable to continuous operation.
Other objects of this invention will be apparent to the reader from the description and examples below.